1. Technical Field
The present disclosure generally relates to a method for controlling aircraft time of arrival, for example to control the time of arrival of the aircraft at a flight trajectory waypoint.
2. Description of the Related Art
Traditionally, most commercial aircraft have a Required Time of Arrival (RTA) function built into the flight control systems of the aircraft. The RTA function controls the altitude and speed so that the aircraft reaches a target waypoint (or a plurality of target waypoints) at a commanded time (or times) known as Required Time(s) of Arrival (RTA). For instance, Scheduled Time(s) of Arrival (STA) at certain target waypoint(s) may be established by an arrival management (AMAN) system for each aircraft arriving to a particular airport, so that aircraft are suitably separated in space and time between each other at each of the target waypoint(s). STAs may also be established by an Airline Operating Center so that the airline orchestrates the arrivals of its flights. Furthermore, pilots themselves may schedule arrival times of their election in some occasions. For instance, they may advance arrival times in order to overcome flight delays, and so force the aircraft to adopt faster speeds.
A target waypoint and its corresponding RTA may be either manually inputted to the flight management computer (FMC) of the aircraft or, alternatively, may be automatically uploaded. In each case, an RTA that is equal to the STA is inputted to the FMC. In the exemplary case that the aircraft operates under AMAN supervision, it is required to take necessary measures to reach each waypoint at the AMAN mandated STAs. For example, the trajectory may be altered by adjusting the aircraft speed, stretching the aircraft flight path, staying in a holding pattern, and so forth.
RTA control in existing commercial aircraft is achieved through an iterative determination of an Estimated Time of Arrival (ETA) of the aircraft at the target waypoint. When the ETA falls outside of an acceptable range of values around the RTA, the FMC searches for a new trajectory that implies an ETA equal to the RTA at the target waypoint (within a given small tolerance). The maximum value of acceptable |RTA−ETA| error is referred to herein as Difference Threshold (DT).
Presently, the appropriate trajectory is identified on the basis of a single coupling variable such as Cost Index (CI), applied across the various stages of the flight (climb, cruise, descent). Cost Index is a numerical parameter that is indicative of a ratio of the cost of the aircraft being in the air (the longer the flight, typically the higher the operating costs) versus the cost of fuel while the aircraft is flying. The CI is most easily understood by considering its limits: at CI=0, the FMC calculates the most fuel efficient trajectory possible, regardless of how long the flight will then take. For maximum CI, on the other hand, the FMC mandates maximum flight envelope speeds, regardless of fuel cost. Hence, CIs between these extremes define different trade-offs between fuel costs and flight times.
Different AMANs, whether operative or still in conceptual or development stages, consider in one way or another a horizon at which the AMAN freezes the STA calculation. The distance of this horizon to the arrival airport typically ranges from 200-300 nautical miles (NM). Along with this, for a high altitude (>30,000 ft) cruising flight, the distance from top of descent (TOD) to touchdown at the airport may be around 100-150 NM. Also, the target waypoint such as an initial approach fix (IAF) may be around 50 NM from touchdown. Thus, current RTA guidance strategies may include anywhere between 50 and 200 NM of aircraft travel at a cruising altitude and 50-100 NM of descent to the airport.
FIG. 1A illustrates an exemplary flight trajectory (vertical profile) of an aircraft from a waypoint (d=zero) during the cruise phase, through to arrival at a destination airport in excess of 300 NM later. TOD is some 125 NM from the airport.
FIG. 1B illustrates, for the same flight trajectory, a time deviation (vertical axis) between the Actual Times of Arrivals (ATAs) and the initially Estimated Times of Arrivals (ETAs) at every simulated point along the trajectory. This deviation is not the same as the (RTA−ETA) error calculated for the target waypoint. It is a variable that indicates how the aircraft is deviating from the initially predicted trajectory. It will be noted that the deviation (which is zero at the initial waypoint in the cruise phase) increases over the course of the cruise phase reaching a maximum of TOD. In part, this (uncorrected) drift may be a result of wind and temperature prediction errors that affect the aircraft groundspeed. This in turn results in a temporally unpredictable time difference shift. A significant correction to the descent speed/altitude is then needed in order to arrive at the target waypoint on time, which is inefficient in time and/or fuel.
Accordingly, there is a need for an improved method for controlling the aircraft so that the aircraft better follows the commanded flight trajectory, that is, the absolute values of the ATAs minus the ETAs are minimized throughout the flight.